1. Field of the Invention
The present invention relates generally to image reading apparatuses and, more particularly, to an image reading apparatus for carrying out a correction of altering a read image signal to an output signal to obtain a suitable print, particularly a white balance correction and a shading correction.
2. Description of the Related Art
An image reading apparatus has conventionally been employed as means for inputting an image of a computer or means for reading an original image of a digital type copier. In such an apparatus, a still image such as an original is read by an image sensor, and resultant image data is then subject to various image processings to output an image signal.
As an optical system of such an image reading apparatus, there is provided in general an equal-scale magnification type optical system for shifting one-dimensional image sensor (a line sensor) comprising a light source for illumination, a rod lens for converging a reflected light from the original, CCDs (Charge Coupled Devices) arranged in the direction of main scanning and the like, in the direction of sub-scanning below a platen glass on which the original is set. In an image reading apparatus for reading a color image, a color separating filter is provided for separating three primary colors, i.e., R (red), G (green) and B (blue) on a front face of a photo detecting area corresponding to each pixel of the line sensor.
A photoelectric conversion output of each of the colors transmitted from the line sensor which reads an original image as separated into the three primary colors is appropriately amplified. This photoelectric conversion output is digitized by analog/digital (A/D) conversion means, and image data based on an intensity of a reflected light of each color in each pixel is generated.
The image data is subjected to numerous image processings and then transmitted as an image signal to an image forming apparatus such as a printer.
In image processing, normalization of image data, called a white balance correction, is carried out between the image reading apparatus and the image forming apparatus in order to correctly reproduce a tone of an original color image. That is, a reference color (normally white) is determined, and thus the image data is normalized so that a relative ratio of the respective colors (R, G, B) is fixed when the original image with a uniform reference color is read. This normalization enhances a compatibility with various types of image forming apparatuses. Furthermore, this normalization enables a correct tone reproduction even in the cases of a replacement of the line sensor, a dispersion in light receiving sensitivity of the line sensor in mass production of the image reading apparatus, and a change with time in the optical system.
In the conventional image reading apparatus, the white balance correction is carried out at the stage of digitization of the photoelectric conversion output. That is, an analog reference potential to be applied to an A/D converter is adjusted in digitization for each color so that the image data of each color becomes uniform when reading a reference color image.
The A/D converter need be provided for each of those primary colors since it is totally impossible to adjust a reference potential for each color in accordance with read scanning of the respective colors to be carried out almost simultaneously, in a fast image reading apparatus. Further, the correction involves analog processing, so that an accuracy in correction is easily affected by an external factor such as temperature.
In automation of the white balance correction by employing a CPU (Central Processing Unit), in particular, it is indispensable to provide a digital/analog (D/A) converter for generating a reference voltage in accordance with data showing a result of arithmetic operation of the CPU, in addition to the A/D converter. Consequently, the configuration of a correction apparatus is complicated.
Further, since a solid image pickup element such as CCD involves a limitation in size due to the size of a semiconductor wafer, the line sensor comprises a plurality of CCD chips in an image reading apparatus for reading images of A3 and A4 in size. Thus, a line sensor consisting of a 5-chip configuration, for example, requires three sets of A/D converters and D/A converters for each chip, i.e., totally 30 converters (3 colors.times.5 chips.times.2 kinds), resulting in a large-scale and expensive apparatus.
Meanwhile, a uniformity of image data, called a shading correction, is effected in order to correct a dispersion in image data due to a sensitivity difference between image pickup elements, a light intensity distribution (an unevenness in quantity of light) of a light source in the main scanning direction, a distortion in a lens and the like. That is, before reading the original, a reference color image with a uniform density is previously read, and reference image data corresponding to one line is stored. Thus the image data is corrected for each pixel in accordance with the reference image data in sending/receiving an image signal for the original.
FIGS. 1 and 2 are block diagrams showing schematic configurations of shading correction circuits S1 and S2, employed in the conventional image reading apparatuses.
The shading correction circuit S1, shown in FIG. 1, is based on a so-called table index method and comprises an RAM 71 for storing reference image data SD7, and an ROM 72 as a shading correction table in which image data Do7 for correction, previously prepared, is written.
The RAM 71 writes the reference image data SD7 corresponding to one line before reading the original. The RAM 71 outputs the reference image data SD7 which is previously read by the image pickup element in synchronization with inputting of the image data Di7 which is read from an original by the same image pickup element.
Addressing of the ROM 72 is carried out by the image data Di7 and the reference image data SD7, so that the ROM 72 outputs the correction image data Do7 of a designated address.
The shading correction circuit S2, shown in FIG. 2, employs a reciprocal coefficient multiplication method and comprises a RAM 81 for storing reference image data SD8 or reciprocal coefficient data ID8, reciprocal operation means 82 for generating the reciprocal coefficient data ID8 in response to the reference image data SD8, and a multiplier 83 for multiplying image data Di8 and the reciprocal coefficient data ID8.
The RAM 81 stores the reference image data SD8 corresponding to one line before reading the original.
The reciprocal operation means 82 repeats reading out reference image data SD8 corresponding to one pixel stored in one address of the RAM 81, generating reciprocal coefficient data ID8 corresponding to the read-out reference image data SD8 and writing the generated reciprocal coefficient data ID8 into the RAM 81. As described above, the reference image data SD8 is replaced by the reciprocal coefficient data ID8 for the content of the RAM 81.
The reciprocal coefficient data ID8 responds to a number resulting from reciprocal conversion of the reference image data SD8 as a maximal data to 1. The image data Di8 is multiplied by a coefficient by the arithmetic operation of the multiplier 83. That is, correction image data Do8 which is corrected to be in a relative ratio to the maximum data of the reference image data SD8 is outputted.
In accordance with the table index method as shown in the shading correction circuit S1 of FIG. 1, a change in the content of the correction image data Do7 which is previously prepared in the ROM 72 enables setting an arbitrary correction pattern and thus an optimal shading correction for various factors of a dispersion in image data generated between pixels on one line. However, the fast-operated ROM 72 with a large capacity is required therefor.
Assuming that the image data is of 8 bits (256 gradations), for example, since a bit scheme of the reference image data must also be 8 bits to obtain a maximum correction accuracy, the ROM 72 is required to have one address of 16 bits and a capacity of 64K byte (64K.times.8 bits). However, such an ROM integrated circuit device with a large capacity and an access time equal to or less than 50 nsec is not propagated for general purpose. This specially ordered ROM product is considerably expensive, so that an ROM with a capacity of 8K byte is unintentionally employed at present. The shading correction is carried out in this ROM of 8K byte by designating one address of 13 bits by image data of 7 bits (128 gradations) and reference image data of 6 bits.
As described above, in the case of employing the table index method, the number of bits of image data is limited by performance of the ROM, so that a transmission of an image signal with a high gradation cannot be realized. Further, when the number of bits of the reference image data is smaller than that of the image data, a correction range is also narrowed. That is, when the number of bits of the reference image data is less than that of the image data by one bit as in the above case, the correction range is limited to the case that the intensity of the image data is larger than half that of the maximum data, while a correction error occurs in the case that the intensity of the image data is smaller than half that of the maximum data.
Meanwhile, the reciprocal coefficient multiplication method requires no memory with a large capacity, and is suitable for a normal shading correction in which the intensity of a reflected light is almost proportional to the image data.
However, in the shading correction circuit S2 of FIG. 2, since an operation of reciprocal coefficient data corresponding to pixels by one line is carried out before the transmission of the image signal, a long time period is required from reading a reference color image to inputting the image data for the original image, and thus a rapid transmission of the image signal cannot be carried out. Further, dedicated control means (CPU) for controlling the arithmetic operation of the reciprocal coefficient data and the replacement of the content of the RAM 81 is required, resulting in a correction apparatus with high cost and a complicated configuration. Particularly, in the image reading apparatus for reading a color image, since the arithmetic operation of the reciprocal coefficient data is carried out for each separated color of the three primary colors, a triple arithmetic operation time is required. In order to prevent multiplication of this operation time, three sets of the CPUs are required, resulting in a disadvantage in cost.